Introduction

The Trichoptera (caddisflies) belong to the
infraclass Neoptera, division Endopterygota; their wings develop
internally instead of externally in wingpads. They are closely related
to the Lepidoptera (butteflies and moths), an insect order with very
few aquatic species. The larvae of all the species of Trichoptera are
aquatic, except for a few cases.

The wings of Trichoptera are
covered with setae, from which the name of the order is derived (from
the Greek "trichos"= hair, and "pteron"=wing).

Three superfamilies have been
distinguished, which include five groups based on case-building
behaviour. Hydropsychoidea (Hydropsychidae, Philopotamidae,
Polycentropodidae, and Psychomyiidae) are the net spinners and retreat
makers; Rhyacophiloidea include the free-living forms (Rhyacophilidae),
saddle-case makers (Glossosomatidae), and purse-case makers
(Hydroptilidae); and Limnephiloidea are the tube-case makers.

Life History

Caddisflies are holometabolous insects whose
eggs are deposited in gelatinous matrices in or out of water. Larvae go
through 5 instars (rarely 6 or 7). Most species are univoltine, but
some complete more than 1 generation per year, whereas others require 2
years for development. The pupal phase is generally 2 to 3 weeks,
although some species may overwinter as pupae. Many caddisflies undergo
a diapause phase, which, depending on the species, occurs in one of
several life history stages. Pupae have functional mandibles that they
use to chew their way out of the pupal case once they are ready to
emerge as adults. Adults are mostly crepuscular, limiting their
activity to dusk or darkness. Adults live from a few weeks to several
months, depending on the species and the nature of the habitat.

Mass emergences of some
species from large rivers are considered a nuisance by residents, since
the insects are attracted to outdoor lights; human allergies to the
scales on their wings have also been reported. The larvae of some
leptocerids are reported to damage the young shoots of rice plants in
paddy fields. The larvae of a few species are known to eat fish eggs.
On the beneficial side, many hydropsychids prey on black fly larvae.

Many species that inhabit temporary ponds
lay eggs in the basins of dry pools in the fall, when the soil surface
is beginning to become more moist. After larvae hatch, they may remain
within the gelatinous matrix for months until the pool basin is
reflooded. Moisture is the apparent stimulus for larvae to break out of
the gelatinous matrix and begin case building or net spinning.

Adult caddisflies differ from moths in a
number of aspects foremost amongst which are patterns of wing venation
and structure of the mouthparts. Adult caddisflies are small (1.5 mm
body length) to moderate-sized (4.0 cm), tend to be drab in colour, and
are mostly active at night, especially around lights. During the day
they hide in riparian vegetation. The compound eyes are well developed
and there may be up to three ocelli. The mouthparts are weak and are
capable only of ingesting liquids. All three thoracic segments are
distinct. The legs are long and slender.

Caddisfly larvae are quite similar to
lepidopteran caterpillars but have only a single pair of abdominal
prolegs which are located on the terminal segment and are each equipped
with an apical anal claw. The larval thorax is well developed, with at
least the pronotum covered dorsally by a pair of sclerotized plates.

The trichopteran pupa is exarate and, in
case-building species, develops within the larval case after it has
been secured to the substrate and sealed with silk. In free-living
species, the final instar larva builds a special pupal case, generally
made from silk and mineral particles, again this is firmly attached to
the substrate. In most species, the pupa is equipped with heavily
sclerotized mandibles which enable it to cut an opening in the case so
that it may escape and swim to the water surface.

Habitat and Distribution

Like mayflies and stoneflies, caddisflies
probably evolved in cold, fast-flowing streams, since families with
more primitive characteristics (e.g., Rhyacophilidae) are restricted to
those habitats. It has been hypothesised that the use of silk for case
construction enabled the Trichoptera to become more diverse
ecologically, providing a respiratory mechanism whereby habitats with
higher temperatures and lower dissolved oxygen levels could be
exploited.

At present, caddisflies inhabit a
wide range of habitats from the ancestral cool streams to warm streams,
permanent lakes and marshes, and permanent and temporary ponds. One
species has been found in tide pools off the coast of New Zealand; the
females oviposit through the papillar pores of starfishes. Caddisflies
have been generally classified as clingers, sprawlers, or climbers,
although a few are burrowers.

Although caddisfly larvae are found in a
wide range of aquatic habitats, the greatest diversity occurs in cool
running waters. Furthermore, in families represented in both lotic and
lentic habitats, the genera exhibiting more ancestral characters tend
to be found in cool streams whereas those showing more derived
characters tend to occur in warm, lentic waters. These two findings
point to cool, running waters as the most likely primordial caddisfly
habitat, the one in which the ancestors of the Trichoptera first became
aquatic and the one in which differentiation into the basic groups
(superfamilies) took place (Williams & Feltmate, 1994).

Restricted to Australian Region;
embryogenesis takes place in coelom of starfishes, larvae become free
living in coastal waters (marine)

Tasimiidae

Australian & Neotropical regions only; clear mountain streams

Limnephilidae

Mostly in cooler parts of Nearctic &
Palaearctic, some in adjacent Oriental, some in temperate Neotropics
(Dicosmoecinae), a few in Australian & Afrotropic regions; most
types of running & standing waters, including temporary &
brackish waters

A final point worth mentioning about the
Trichoptera, in general, and about those that live in small streams in
particular, concerns habitat specificity. Many species that are
restricted to small streams reflect the ecological characteristics of
the surrounding terrestrial community. In such streams, conditions for
the larvae are affected by shade in summer or winter, the amount and
periodicity of leaf-fall, and the distribution of local precipitation,
all three of which are integrated with the type of climax community
occupying the general area. As a result, there is, in general, a marked
correlation between these terrestrial biomes and the ecological
affinities of their respective small-stream caddisfly faunas. Such
correlations are of enormous importance to palaeoecology, and caddisfly
remains are proving to be a powerful tool in the interpretation and
reconstruction of past environments (Williams & Feltmate, 1994).

Feeding

Caddisfly larvae occupy every conceivable
trophic level or functional feeding group. Many Limnephiloidea are
shredders or grazers, and Hydropsychoidea are characteristically filter
feeders or predators, using silken nets to collect seston or catch
prey. Caddisflies can also be selective feeders, preferentially
removing more-nutritious foods from their nets (algae, animals). Some
Limnephiloidea filter feed (Brachycentridae: Brachycentrus)
by orienting their legs into the current to trap particles suspended in
the water column. The saddle- and purse-case makers are specialized
algal grazers and can defend territories of rich algal resources or
depress algal densities, thereby outcompeting other grazers. The
free-living forms and a few of the tube-case makers feed on other
insect larvae, crustaceans, or annelids.

Indicator value

Like mayflies and stoneflies, caddisflies
probably evolved in cold, fast-flowing streams, since families with
more primitive characteristics (e.g., Rhyacophilidae) are restricted to
those habitats. It has been hypothesised that the use of silk for case
construction enabled the Trichoptera to become more diverse
ecologically, providing a respiratory mechanism whereby habitats with
higher temperatures and lower dissolved oxygen levels could be
exploited.

At present, caddisflies inhabit a
wide range of habitats from the ancestral cool streams to warm streams,
permanent lakes and marshes, and permanent and temporary ponds. One
species has been found in tide pools off the coast of New Zealand; the
females oviposit through the papillar pores of starfishes.

Although caddisfly larvae are
found in a wide range of aquatic habitats, the greatest diversity
occurs in cool running waters. Furthermore, in families represented in
both lotic and lentic habitats, the genera exhibiting more ancestral
characters tend to be found in cool streams whereas those showing more
derived characters tend to occur in warm, lentic waters. These two
findings point to cool, running waters as the most likely primordial
caddisfly habitat, the one in which the ancestors of the Trichoptera
first became aquatic and the one in which differentiation into the
basic groups (superfamilies) took place. (Williams and Feltmate, 1992)

Mass emergences of some
species from large rivers are considered a nuisance by residents, since
the insects are attracted to outdoor lights; human allergies to the
scales on their wings have also been reported. The larvae of some
leptocerids are reported to damage the young shoots of rice plants in
paddy fields. The larvae of a few species are known to eat fish eggs.
On the beneficial side, many hydropsychids prey on black fly larvae.

Some physiological and ecological tolerances and requirements (Mackie, 2001)